This invention relates to a vibration damper. More specifically but not exclusively this invention relates to an intershaft bearing damper where the bearing is situated between two rotating shafts. More particularly still this invention relates to a bearing damper suitable for use in a gas turbine engine.
Gas turbine engines in particular need to comply with rigorous safety standards. Vibration reduction is an essential part of safety improvement for gas turbine engines. Some gas turbine engines employ a concentric shaft arrangement with intershaft bearings provided between the shafts. One example of a concentric shaft arrangement is where the high pressure turbine of a gas turbine engine is mounted on a shaft concentric with the intermediate pressure turbine shaft, each turbine comprising a number of nozzle guide vanes (NGVs).
An arrangement of concentrically mounted and contra-rotating shafts produces vibration and a need has been identified for control of such vibration.
An intershaft bearing positioned between two rotating shafts is subject to radial loads known to cause the vibration problems in gas turbine engines. In order to try and limit the effect of such dynamic loads from the rotating assemblies to the bearing housings a xe2x80x98squeeze filmxe2x80x99 type of bearings may be employed.
Such bearings have a small clearance between the outer race of the bearing and its housing, the clearance being filled with oil. The effect of the oil is to dampen the radial motion of the rotating assembly and the dynamic loads transmitted to the bearing housing thus reducing the vibration level of the gas turbine and hence the possibility of damage by fatigue.
However squeeze film damping of a bearing normally requires a pressurised oil supply of typically 100 psi. This supply is metered through a control orifice such that the squeeze film surfaces are fed with oil at typically 20 psi.
The problem of vibration control with intershaft bearings is associated with the need to supply pressurised oil to the squeeze film surfaces across the resultant junctions between the rotating shafts. A sealed oil supply operating at the above typical pressures would require extremely complex sealing arrangements which may inevitably prove to be unreliable, costly and heavy. It is also likely that the safety regulations for gas turbine engines would require such a potentially unreliable sealed pressurised system to be totally separate from the existing bearing lubrication system.
There is a need, therefore, for improved bearing damping which attempts to alleviate the aforementioned problems.
According to the present invention there is provided a vibration damper comprising a bearing located between two rotatable shafts, said bearing comprising an inner race and an outer race said outer race being located adjacent a bearing housing,
An axially extending oil entry surface positioned radially inwardly of the radial outer surface of said outer race and axially adjacent one end of said bearing, the outer surface of said bearing outer race forming a clearance between itself and the bearing housing suitable for the ingress of a film of oil directed from said oil entry surface, a weir positioned at the axially opposite end of said bearing, said weir being radially inward of said bearing outer race and said oil entry surface whereby in use the radial height difference therebetween provides a pumping force thus supplying a film of oil between the bearing outer race and its casing.
Preferably said rotatable shafts are mounted concentrically.